1. Field of the Invention
Aspects of the present invention relate to a fusing unit and an image forming apparatus including the same, and more particularly, to a fusing unit having a high thermal efficiency and an image forming apparatus including the same.
2. Description of the Related Art
In general, a fusing unit performs a fusing process in an electrophotographic type image forming apparatus, which forms an image on a printing medium by performing a series of processes, including exposing a toner image, developing the toner image, transferring the toner image to the printing medium, and fusing the toner image. The fusing unit fuses toner, which is initially applied to the printing medium in a liquid state, onto the printing medium by heat and pressure so that the toner hardens on the printing medium.
As shown in FIG. 1, a conventional fusing unit 10 includes a pair of rollers including a heating roller 13 and a pressing member 17. The heating roller 13 and the pressing member 17 include cylindrical metal rotating bodies 131 and 171 which are rotatably supported by bearings 135 and 175, respectively, and elastic layers 132 and 172 formed of an elastic material on a circumference of the metal rotating bodies 131 and 171, respectively. Also, halogen lamps 133 and 173 are located inside the metal rotating bodies 131 and 171 and held in position in hollow glass pipes 134 and 174, respectively. A fusing nip surface, which is a common contact area where the heating roller 13 and the pressing member 17 contact each other, is heated to a predetermined fusing temperature by radiant heat generated by the halogen lamps 133 and 173. The halogen lamps 133 and 173 are supplied with electric power by electrode brushes 136 and 176, respectively.
However, since the conventional fusing unit 10 has airspace F and the elastic layers 132 and 172 located between the fusing nip surface and the halogen lamps 133 and 173, heat is lost during the fusing process, thereby lowering thermal efficiency.
To improve thermal efficiency, an induction heating type fusing unit, which uses an induction coil in addition to the halogen lamps 133 and 173 used by the above-described lamp heating type fusing unit 10, has been developed. The induction heating type fusing unit disclosed in Korean Patent Publication No. 2005-60488 has a more improved thermal efficiency than the lamp heating type fusing unit 10 described above. However, the induction heating type fusing unit heats an entire circumference of a heating roller, even though only a fusing nip portion is used to fuse the toner to the printing medium, thereby lowering thermal efficiency. Accordingly, when the induction heating type fusing unit is heated up, the entire circumference of the heating roller is heated, which has a large thermal capacity. As a result, the induction heating type fusing unit takes a long time to reach a fusing temperature.